https://en.wikipedia.org/wiki/Egress_filtering
If every large network operator did this, then shutting down abusive hosts used to generate the kind of traffic needed for attacks like these would be possible.
You'd make a list of all the IPs for each large provider from which excess traffic is coming, and you'd send them to each large provider, and they'd either block the IPs completely (requiring customers to fix their compromised machines before they could get back on the Internet), or at very least they would block all traffic to the destination of the attack.
Some silly people think this isn't possible, but if large providers are forced to do it, then they can easily tell smaller ones they'll be blocked if they don't also do this, and so on, and we'd have a slightly more responsible Internet again.
Too bad we have shitty, huge companies that don't care about doing the right thing :(